Fig. 4. Differences in aerodynamic force production between the performance of a single flapping wing (b) and when simultaneously flapping an imaged wing (a) at close distance to elicit a clap-and-fling wing beat. (A) Difference in total force production (a–b) normal to the wing surface due to wing–wake interaction are plotted in black; differences in lift production are shown in red and differences in drag are plotted in blue. The time traces demonstrate that dorsal wing interaction due to clap-and-fling may augment but also diminish force and lift production throughout the entire stroke cycle. Roman numbers (I–VI) label the main force peaks found in the data traces. (B) The small black dots during clap-and-fling indicate the times in fractions of the stroke cycle at which digital particle image velocimetry (DPIV) was performed. The stroke cycle starts and ends at =1.00 (dorsal wing excursion); however, due to the employed kinematic pattern the two wings clap (wings are held in parallel) about 1% or 50 ms after the beginning of the downstroke.